Abstract

A high frame-rate near-infrared (NIR) tomography system was created to allow transmission imaging of thick tissues with spectral encoding for parallel source implementation. The design was created to maximize tissue penetration through up to 10 cm of tissue, allowing eventual use in human imaging. Eight temperature-controlled laser diodes (LD) are used in parallel with 1.5 nm shifts in their lasing wavelengths. Simultaneous detection is achieved with eight high-resolution, CCD-based spectrometers that were synchronized to detect the intensities and decode their source locations from the spectrum. Static and dynamic imaging is demonstrated through a 64 mm tissue-equivalent phantom, with acquisition rates up to 20 frames per second. Imaging of pulsatile absorption changes through a 72 mm phantom was demonstrated with a 0.5 Hz varying object having only 1% effect upon the transmitted signal. This subtle signal change was used to show that while reconstructing the signal changes in a tissue may not be possible, image-guided recovery of the pulsatile change in broad regions of tissue was possible. The ability to image thick tissue and the capacity to image periodic changes in absorption makes this design well suited for tracking thick tissue hemodynamics in vivo during MR or CT imaging.

Figures (7)

A diagram of the video-rate near infrared tomography system is shown. Eight spectrally encoded LD systems were integrated onto one cart [5]. Eight fiber-coupled spectrometers with high-resolution CCDs were integrated onto another cart and set to respond to an external TTL trigger signal. The signal was generated by a data acquisition board and then split into eight channels by a customized splitter circuit, and delivered to the EXT SYNC port on each CCD simultaneously. Imaging data are transferred back to the computer for post processing. A flexible fiber mount was customized to hold the phantom or tissue in between.

Direct reconstruction results of µa on the imaging plane of a heterogeneous phantom with different blood concentrations in the inclusion. Figures 2(a)–2(d) showed phantoms with swine blood concentration of 1%, 2%, 3% and 4%. The reconstructed and true values of µa in the inclusion of different swine blood concentrations were listed in Fig. 2(e), and plotted in Fig. 2(f) together with the reconstructed average value of background µa.

Difference reconstruction results of µa on the imaging plane of a heterogeneous phantom with different blood concentrations in the inclusion. Figures 3(a)–3(d) showed phantoms with swine blood concentration of 1%, 2%, 3% and 4%. The reconstructed and true values of difference µa in the inclusion of different swine blood concentrations were listed in Fig. 3(e), and plotted in Fig. 3(f) together with the mean value of background µa.

Results of dynamic blood diffusion phantom experiment. Figure 4(a) showed the direct reconstructed µa of both ROI and background versus time. Figure 4(b) shows the difference reconstructed µa value of both ROI and background versus time. Curve µa_ROI is the mean value of µa in the inclusion, while curve µa_BKG is the mean value of µa of the background. 10 mW LDs were used as source beam. 100 ms exposure time and 100 µm slit width were set up at all spectrographs. Images were acquired at 20 frames per second.

System setup of the pulsatile phantom experiment. High µa solution was continuously pumped through a balloon at 0.5 Hz. The balloon was submerged in a 72 mm thick slab container filled with low µa solution. The absorption contrast of the solution inside the balloon against outside was 3 to 1. Eight LDs were launched as sources, and 6 spectrometer systems were set as detectors to acquire data at 10 frames per second. Slit widths on all spectrographs were set to 100 µm.

Signals of different S-D pairs in frequency domain. Figure 6(a) is detected signals of 3 different optical paths in frequency domain after being normalized to their individual mean intensity. Only 0 to 1 Hz of these spectra were displayed. Figure 6(b) illustrates the 3 different optical paths through the phantom with straight lines. D1 to D6 were 6 detection spots connected to spectrographs through optical fibers. S1 to S8 were 8 laser beam input spots connected to LDs through optical fibers.

Reconstructed µa of the ROI in the time-domain and frequency-domain. In Fig. 7(a) the mean recovered µa value in the ROI is plotted versus time with a direct diffuse reconstruction method. In Fig. 7(b) the Fourier transform of the mean µa in Fig. 7(a) is shown. In Fig. 7(c) the mean µa recovered in the ROI versus time is shown using region-guided direct reconstruction methods. Figure 7(d) is the Fourier transformed amplitude data from Fig. 7(c), showing the dominance of the 0.5 Hz signal in this frequency spectrum.